1. Technical Field of the Invention
The present invention relates generally to the field of integrated circuits and, more particularly, to evaluating a state condition of an anti-fuse.
2. Description of the Related Art
Integrated circuits often require programmable electrical connections between circuit nodes. Such connections can be implemented by use of an anti-fuse link. Anti-fuses are fabricated with a structure similar to that of a capacitor in which two conductive electrical terminals are separated by a insulating layer, like silicon oxide. In an xe2x80x9coffxe2x80x9d state, the antifuse has a high resistance between its terminals. The anti-fuse can be programmed to an xe2x80x9conxe2x80x9d state (i.e., a low resistance) by breaking down the interposed layer to form a conductive link between the anti-fuse terminals.
Electrical anti-fuses have been introduced into semiconductor products and are, in many applications, replacing the commonly used laser fuses. One of the currently applied anti-fuse types is a structure based on a conventional MOS transistor. Such an anti-fuse is blown by applying a blow voltage (generally about 7 Volts) across the gate-oxide of the MOS transistor. The blow process results in a damaged gate-oxide which reduces the electrical resistance across the oxide. An evaluation circuit attached to the anti-fuse must be able to differentiate between the high resistance of the intact oxide (Roff) and the lowered resistance of the damaged oxide (Ron).
Typical values for Roff are in the range of 1012 ohms. The range of Ron largely depends on the parameters during the blow process. High blow voltage and high blow current generally lead to lower Ron resistances. However, it is desired to minimize those two parameters in order to reduce the size of the associated circuits (e.g. blow voltage generator, blow transistor, wiring, etc.). From that perspective, practical values of Ron are in the range of 105 ohms to 108 ohms. Fuse evaluation circuits commonly used for laser fuses and electrical fuses are not be able to accurately distinguish an unblown anti-fuse and a blown anti-fuse exhibiting the relatively high practical Ron values.
Therefore, there is a need for a method and circuit for evaluating the state of a MOS transistor anti-fuse, including a blown anti-fuse exhibiting a relatively high gate oxide resistance.
The present invention achieves technical advantages as an apparatus, system and method of evaluating a programmable anti-fuse element. For a programmable transistor antifuse element, the gate of the anti-fuse is precharged with a predetermined voltage and/or current and the anti-fuse is subsequently evaluated. In one embodiment, a precharge sufficient to provide an operating threshold voltage is applied to the gate. Here, an intact (unblown) transistor remains ON over a period of time and a damaged (blown) transistor dissipates the ON voltage and turns OFF. The status of the transistor is subsequently determined by evaluating the resistance between the drain and source. A high resistance indicates a blown condition and a low resistance indicates an unblown condition. In another embodiment, a small current is provided to the gate in which the small current is greater than a leakage current for an intact transistor and is less than a leakage current for a damaged transistor. An intact transistor charges to an ON state over a period of time but a damaged transistor does not because it""s leakage current is greater than the small current provided to the gate. Again, the status of the transistor is subsequently determined by evaluating the resistance between the drain and source.